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1. Field of the Invention
The present invention relates to an improved gasifier/reactor fed by a biomass of cellulosic material such as granulated wood, rice hulls, chopped cane and the like, for the production of a gas selectively rich in carbon containing components such as carbon monoxide, carbon dioxide, hydrogen and methane which in turn may be converted into a selected end product fuel such as methanol or ethanol, or as a feed gas for an industrial power plant.
2. General Background of the Invention
Gasification of wood, wood chips including sawdust, wood charcoal and other particulate cellulosic materials have become of increasing interest and importance because of the volatility of petroleum prices, dwindling of fossil fuels such as domestic petroleum and natural gas resources, and the increased dependence of the United States on international imports of these fuels. Gasification of coal and biomass has been practiced for over 100 years, and there are many varieties and types of gasifiers and methods of gasification.
In the instance of gasification of wood, wood chips and wood charcoal and other similar biomass fuels for the production of gas rich in combustibles, static grates or the equivalent have been utilized for supporting the fuel bed of progressively carbonized material and distributing the air, steam or other transport gas to support the pyrolysis gasification process. Vessels that have traditionally been used successfully for gasifing granular biomass such as wood chips and similar cellulosic material have been cylindrical, or somewhat wider or narrower at the grate level than at the surface of the fuel bed, according to the flow of feed and the forced air (or other gas) draft. Concerns with the settling of the fuel bed so that combustion takes place without the need to poke or otherwise stir the fuel bed have provoked a variety of vessel construction, none of which lend themselves to well to a high volume, precisely controlled, continuous process wherein the biomass fuel is efficiently converted to the target gas for supply to conversion into the ultimate fuel to be marketed or used. Prior art gasifiers have traditionally been large structures of brick and mortar, including complicated feed, blower and control systems. U.S. Pat. No. 5,551,958 to Antal; U.S. Pat. No. 5,507,846 to Coffman; U.S. Pat. No. 5,486,269 to Nilsson; U.S. Pat. No. 5,226,927 to Rundstrom; U.S. Pat. No. 4,655,891 to Atwood; U.S. Pat. No. 4,498,909 to Milner, et al; and U.S. Pat. No. 4,385,905 to Tuckerare illustrative of the various reactors and processes which have been utilized for the conversion of biomaterials to a syngas. U.S. Pat. No. 1,901,170 to Karrick discloses the use of a helical coil in a closed loop for gasification of coal and coke, however the operation of such a unit with biomass material is unclear. Necessary feed and mixing mechanisms and gasifier structure for the range of temperatures, pressures and through rates are not disclosed or suggested.
Gasification of biomass fuels falls into one or more of the following categories: pyrolysis, air gasification, oxygen gasification and anaerobic digestion. Pyrolysis is the breakdown of the biomass by heat at elevated temperatures (400xc2x0 F. to 1200xc2x0 F.) to yield an intermediate gas which is ultimately transformed into a market fuel (gas or liquid such as methane or ethanol). The intermediate gas produced is dependent upon the feed source and the speed and temperature at which the pyrolysis occurs. Fast pyrolysis of finely divided biomass results in maximum intermediate (synthesis gas) gas yields. Inclusion of such as oxygen or steam during the pyrolysis assists in the production of an intermediate gas containing carbon monoxide, carbon dioxide and hydrogen, useful in later conversion into such as ethanol, methanol, ammonia or methane. Other gas additions such as air or nitrogen may be used for synthesis gas having other make-up required for different end products. Anaerobic digestion may be utilized (usually in a secondary reactor) to facilitate various means for the conversion of the intermediate gas into one or more of these final fuels or products.
The difficulties in gasification is the conversion of all of the elements comprising the biomass fuel into gases containing the highest amounts of energy, for later conversion into the final products and the minimization of ash and char. In certain biomass fuels, gasification at lower temperatures produces oils and char requiring additional processing and likely, additional energy or waste in the process. Exposing the base fuel during the pyrolysis to air, water vapor or other components has a direct impact upon the products of pyrolysis, as does the temperature of the process and the duration thereof. By using any of the processes of the prior art, such as the fluidized bed, which is, at least, initially exposed to air and can be additionally exposed to such as oxygen, or others of the described input gasses, some portion of the fuel for gasification is consumed, as by oxidation (burning) effecting the output of the process by producing ash or other undesirable residue. Likewise, the startling size and complexity of installations for the effective production of synthesis gas are illustrated in the above cited patents.
The present invention provides an improved method and apparatus for producing a synthesis gas from a biomass feed. In particular the present invention incorporates a reactor vessel heated, at least in part, by an external source such as natural gas, and the reactor vessel includes a helical coil of many turns carrying the biomass feed and the transport gas which are disposed adjacent the sidewall of the vessel, allowing an air gap between the coil and the vessel to permit convective heating. The coil receives a feed of biomass material, in ground or granulated form which is mixed and transported through the reactor coil by a transport gas. The transport gas may provide heat and chemical support to the pyrolysis process in addition to the externally supplied heat which transforms the biomass material into a target synthesis gas in the reactor coil. The rate and control over the pyrolysis process in the coil are effected by the inclusion of separated radiant and convective heat zones in the reactor vessel, the zones being defined by a cylindrical heat shield disposed in the vessel, concentrically of the coil, and in the uppermost region of the vessel, above the burner fed by the natural gas. The cylindrical heat shield preferably includes a truncated conical section at the bottom of the cylinder (closed at the end toward the burner) to better establish the transition between the zones and facilitate the convective heating in that zone.
Preferably, the reactor vessel includes a pressurized mixing chamber in which the biomass feed material is mixed and supplied to the reactor coil. Heating of the biomass by a transport gas which also may facilitate the pyrolysis, such as superheated steam enhances the mixing and feed of the biomass through the reactor coil to produce the synthesis gas.
The inclusion of a secondary reactor on the output of the innovative reactor provides further flexibility in the manufacture of the synthesis gas or a product gas or fuel.